FIG. 28 is a circuit diagram of conventional electronic device 1301, a high-frequency filter, used in an electronic device, such as a radio communications apparatus. Electronic device 1301 includes input/output terminals 1302 and 1303, signal line 1304, series arm resonators 1305, and parallel arm resonators 1306. Signal line 1304 connects between input/output terminals 1302 and 1303. Series arm resonators 1305 are connected in series to each other along signal line 1304. Parallel arm resonators 1306 are connected between signal line 1304 and reference potential 1308. Series arm resonators 1305 and parallel arm resonators 1306 constitute a ladder circuit, and function as a band-pass filter having a passband and stopbands.
Conventional electronic device 1301 has a limitation in an attenuation amount in the stopbands, and is prevented from having a large attenuation amount in a particular stopband.
In recent years, downsizing of radio communications apparatuses such as mobile phones continues being advanced. For this purpose, the number of components used in radio communications apparatuses is decreased. In a radio circuit section of such a radio communications apparatus, the number of components can be decreased by reducing inter-stage filters in a transmission signal path and a reception signal path. However, it is necessary to improve an isolation characteristic of antenna duplexers in order to reduce the number of the components.
FIG. 29 is a circuit schematic diagram of another conventional electronic device 1000, an antenna duplexer. Electronic device 1000 includes phase shifter circuit 1006 connected to common terminal 1003, filter 1001 connected to transmitting terminal 1004, and filter 1002 connected to receiving terminal 1005. Both filters 1001 and 1002 are connected to common terminal 1003 via phase shifter circuit 1006. In electronic device 1000, capacitor element 1007, an external circuit, is connected between transmitting terminal 1004 and receiving terminal 1005 provides path 1000A from transmitting terminal 1004 to receiving terminal 1005 via filter 1001, phase shifter circuit 1006, and filter 1002, and another path 1000B from transmitting terminal 1004 to receiving terminal 1005 via capacitor element 1007. Electronic device 1000 can making a difference in phase of about 180 degrees between a signal passing through path 1000A and another signal passing through path 1000B by adjusting an amount of phase shift of phase shifter circuit 1006, thereby canceling these signals passing from transmitting terminal 1004 to receiving terminal 1005 to secure an isolation characteristic.
An electronic device, a conventional antenna duplexer similar to electronic device 1000, is disclosed in, for example, WO2010/073377. The isolation characteristic can be improved according to this literature. The isolation characteristic of electronic device 1000 however may degrade depending on ambient temperatures.
FIG. 30 is a circuit schematic diagram of conventional electronic device 1000 shown in FIG. 29 used as an antenna duplexer mounted to radio communications apparatus 1200. Common terminal 1003 of electronic device 1000 is connected to antenna 1201, transmitting terminal 1004 is connected to transmitter circuit 1202, and receiving terminal 1005 is connected to receiver circuit 1203. A transmission signal generated in transmitter circuit 1202 is input through transmitting terminal 1004, and output from common terminal 1003 and via antenna 1201 as a radio wave. An incoming signal received from antenna 1201 is input to common terminal 1003, and output to receiver circuit 1203 from receiving terminal 1005.
In electronic device 1000, the incoming signal input from receiving terminal 1005 is weak whereas the transmission signal input from transmitting terminal 1004 is strong. A part of power of the transmission signal may leak to filter 1002 and causes an adverse influence on a receiving performance.
FIG. 31 is a circuit diagram of another conventional electronic device 540, a high-frequency filter used as an antenna duplexer in a radio communications apparatus. Electronic device 540 includes piezoelectric substrate 531, filters 511 and 512, antenna terminal 513, unbalanced transmitting terminal 514, a pair of balanced receiving terminals 515A and 515B, ground 516, and inductor 517. Both filters 511 and 512 are formed on piezoelectric substrate 531. Filter 511 has signal line 518 connects from antenna terminal 513 to transmitting terminal 514. Filter 511 includes series arm resonators 519 connected in series to each other along signal line 518 between antenna terminal 513 and transmitting terminal 514. Filter 511 further includes parallel arm resonators 520 connected between signal line 518 and ground 516. Series arm resonators 519 and parallel arm resonators 520 constitute a ladder-type filter. Filter 512 includes one one-port resonator 521, three non-inverting type longitudinally coupled resonators 522 and one inverting type longitudinally coupled resonator 523 that are connected between antenna terminal 513 and each of the pair of balanced receiving terminals 515A and 515B so as to constitute a band-pass filter having an unbalanced-to-balanced converting function.
Conventional electronic device 540 may not have a sufficient isolation between transmitting terminal 514 and each of receiving terminals 515A and 515B, accordingly being prevented from having an adequate attenuation amount in a receiving band.